1. Field of the Invention
This invention relates to a system for controlling the outlet pressures of district regulators in a fluid distribution system proportionally to the demand of the fluid. More particularly, this invention relates to an adaptive system that varies the output pressures of district regulators in natural gas distribution systems proportionally to the demand for gas. The system predicts the demand for natural gas and raises or lowers outlet pressures in anticipation of the demand so as to maintain a relatively constant pressure at points of the distribution system remote from district regulators. The prediction is automatically formulated based on temperature, time of day, and the observed past performance of the distribution system.
The invention includes a distributed intelligence system that adaptively converges to an accurate prediction of natural gas loads. The predictive function is performed by intelligent controllers located at the district regulators. Intelligent low pressure monitors are located at remote points on the gas distribution system. A centralized computer is used to communicate with the distributed controllers and to provide a manual override, but is not necessary for day to day operation.
2. Description of Prior Art
Currently, outlet pressures for district regulators in fluid distribution systems such as natural gas distribution systems are manually set on a seasonal basis. This represents a very coarse prediction of the demand for gas. The outlet pressure is set to the highest value expected to be necessary for the season. The operator chooses the setting based on past history for the particular distribution system. Because the demand can fluctuate greatly within a season, the outlet pressure is higher than optimum most of the time.
Numerous techniques have been employed to achieve real time correlation between the demand for a fluid, such as natural gas, and district regulator outlet pressures. The feedback approach utilizes pressure sensors at low pressure points in the fluid distribution system in constant communication with controllers located at the district regulators. For example, U.S. Pat. No. 3,878,376 teaches a computer operated solenoid valve pressure control system including a computer, equipment which converts the computer instructions to electrical power to operate solenoid valves which, with associated components, pressurize or depressurize a closed volume, and a pressure measuring system which provides feedback to the computer. Although the feedback approach provides control based on real-time demand, it requires the presence of a constant communication channel and, thus, the attendant expenses associated therewith.
An alternative approach which circumvents the need for a constant communication channel is the purely predictive approach which involves placement of a pre-programmed load profile in a controller at the district regulator. The load profile contains the demand for gas versus the time of day and temperature. For example, U.S. Pat. No. 4,200,911 teaches a method and apparatus for optimum distribution of water in which the actual water consumption at selected nodes of a network from which water is supplied to consumers is measured to detect a standard pattern for water demand in each selected area. Predicted demand patterns for each and every node are then determined by comparing the characteristics or attributes of each area with those of areas having standard demand patterns. Manipulated variables of pumps and valves installed in the pipeline network are then controlled on the basis of predicted demand patterns. See also U.S. Pat. No. 4,562,552 and U.S. Pat. No. 4,569,012, both of which also teach predictive approaches. See also U.S. Pat. No. 5,047,965 which teaches a microprocessor controlled gas pressure regulator valve having a spring biased diaphragm controlled pilot valve whereby adjustment is automatically effected by supplying augmenting pressure to the spring side of the diaphragm by means of an electrically adjustable regulator valve under the control of the microprocessor. Historical pressure drop data as a function of day of the week, time of day, and ambient temperature are stored in the microprocessor. The temperature sensor provides the microprocessor with a signal indicative of ambient temperature, and this is correlated with the stored historical data to determine the adjustment for the main regulator valve. The problem with each of these predictive approaches is the requirement that the profile be compiled by an experienced operator and be updated periodically as system loading varies over the long term. Failure to periodically update the profile on a frequent basis can significantly reduce the effectiveness of the predictive approach and, of course, the requirement for a human operator to update the profile adds significantly to the cost of this approach.